A non-geostationary satellite system comprises a group or constellation of satellites that orbit the earth at altitudes other than geostationary orbit (c.a. 36,000 kilometers above the earth's surface). The non-geostationary satellite systems that are in low earth orbit (LEO) have less propagation loss and less propagation delay than geosynchronous satellite systems due to the lower orbit of the non-geostationary satellites. Such satellites are, therefore, better suited than geostationary satellites for interactive communications, such as internet services.
Geostationary satellite systems have an orbital period equal to the rotation period of the Earth and therefor appear, from Earth, to be at a fixed position in the sky. Non-geostationary satellites move at relatively higher speeds and therefore appear to an earthbound observer to pass overhead from horizon-to-horizon. Because of this relative movement between non-geostationary satellites and the Earth, such satellites move in and out of range of earth-bound user terminals. Such terminals must therefore switch their communications link from one satellite to the next (i.e., hand-off) to achieve continuous communications.
In some systems, radio transmissions from the satellite to the user terminal are in the form of multiple independent beams aimed in different directions. So, in addition to earth-bound user terminals contending with the hand-off between satellites, there is a hand-off between individual beams of an individual satellite, as the coverage area of a satellite moves past a particular user.
The data throughput of such a satellite system, separate from any EPFD (equivalent power flux density) and waveform question, is a function of power and MHz/Km2 of coverage. Although power can be used to increase the spectral efficiency, given as “bits/sec/Hz of bandwidth (bps/Hz), optimally enough power is applied to operate in the most efficient bits/hz/sec of QPSK (quadrature phase shift keying). If power is algorithmically set to that which achieves QPSK per the link budget, the next parameter to adjust to get more bits of data to the ground is to increase the MHz/Km2.